1. Field of the Invention
The present embodiments relate to lightning protection systems in general, and in particular, to a system for protecting composite-body aircraft from damage caused by lightning strikes.
2. Related Art
It is well accepted that composite (typically, carbon- or graphite-fiber-and-epoxy-resin matrices) structural components currently being incorporated into the bodies of advanced types of aircraft will be subjected to naturally occurring lightning discharges, or “strikes,” during flight. In a typical lightning strike incident, the lightning strikes, or “attaches,” at one extremity of an aircraft, and departs, or “detaches,” from another, resulting in a very large, momentary flow of electrical current through the body of the aircraft between the two points.
The more severe, or “primary,” strikes tend to attach to and detach from the body of the aircraft at features that are located at or near protuberances at the extremities of the body (e.g., the nose, tips and leading edges of wings, stabilizers, vertical fins and rudders, engine intake cowlings and diverters, and the trailing edges of rudders, elevators, and ailerons), and are characterized by a fast-rising, high-peak current (2×105 amp) and a large energy transfer density (2×106 amp2 sec) having frequency components of from between about 1 kHz to 1 MHz. These strikes can cause severe structural damage to unprotected composite structures and their contents, as compared to conventional aluminum aircraft structures, because the energy of the strike cannot be efficiently conducted through the composite material without damage, due to its relatively lower thermal and electrical conductivities.
Additionally, other “secondary” parts of the structure, located between the typical primary attachment and detachment points, can be subjected either to primary, or to lesser discharges, referred to as “swept-stroke” lightning strikes. The latter are characterized by a fast-rising current having the same frequency spectrum, but with 1×105 amp peaks and energy transfer densities of 0.25×106 amp2 sec, and can also result in severe structural damage to unprotected composite structures. The probability of experiencing either type of strike at a secondary part of the aircraft appears to be increased for aircraft that have smoothly changing shapes, such as the family of so-called “Blended-Wing-Body” (“BWB”) aircraft currently under development or in production, including the B-1 and B-2 “stealth” bombers and the F-117 stealth fighter. These aircraft are characterized by smooth, blended shapes that make extensive use of composite body structures and other stealth measures to defeat their detection by radar and the like.
To prevent or minimize damage to a composite aircraft resulting from either type of strike, it is necessary to connect the attachment and detachment points of the strike with a continuous, highly conductive path that is capable of carrying a momentary, high-density electrical current without damage, such that the electrical current of the strike is substantially diverted through the conductive path, rather than through other portions of the aircraft that cannot tolerate such a current flow without damage.
Prior systems to effect lighting protection for composite aircraft structures have centered around the provision of metal coated or plated fibers in the plies of the composite structure, as described in U.S. Pat. No. 4,502,902 to Bannik, Jr. et al., or by applying a knitted wire covering over the body of the aircraft, as described in U.S. Pat. No. 3,755,713 to Paszkowksi, or by bonding a thin, metallic foil to the surface of the aircraft, as described in U.S. Pat. Nos. 5,127,601 and 5,352,565 to Schroeder. While these systems do provide some measure of protection against damage occasioned by lightning strikes, they do so at the cost of increased complexity and expense, increased weight, and/or a reduced structural integrity of the composite material. For example, a lighting protection system that relies on metal-plated graphite fibers both substantially increases the weight, and decreases the strength of the resulting composite structure.
What is needed then, is a simple, light weight, low-cost approach for the protection of a composite-body aircraft against direct lightning strikes that may occur at any location on the aircraft surface.